Situation

Lettuce is an intensively managed speciality crop for which there is
little tolerance for weeds. Weed problems are greatest during stand establishment
when they are difficult to remove and can provide competition early in
the development of the crop that can reduce yield, quality and delay harvest.
Weeds can also serve as a reservoir for insect pests. Balan® (benefin),
Kerb® (pronamide), and Prefar® (bensulide) are herbicides that
are commonly used preplant and preemergence to control weeds during lettuce
stand establishment. Lettuce has good tolerance to these herbicides when
growing conditions are favorable and the herbicide's label directions
are followed. However, crop injury can occur under some conditions from
any of these herbicides. Growers must be willing to accept a reasonable
risk of slight crop injury if they wish to control weeds with the preemergence
herbicides available today. This bulletin is intended to help identify
the causes and symptoms of herbicide injury in lettuce from Balan®,
Kerb®, and Prefar®.

Causes of Injury

Crop Vigor and Environmental Stress

Lettuce has the ability to tolerate Balan®, Kerb®, and Prefar®
when it is growing vigorously and exposed to safe levels of these herbicides.
Injury may occur when the crop is weakened or exposed to harmful herbicide
levels.

Crop vigor is dependent upon many factors. Proper irrigation, fertility
and pest management along with good variety and seed selection are necessary
to maintain vigorous growth. Environmental stresses are not often possible
to control. Temperature extremes, excess precipitation, wind and other
adverse climatic conditions can all place stress upon the crop. Lettuce
is sensitive to salt injury which can stress the crop and predispose it
to herbicide injury. The ability of lettuce to tolerate herbicides, especially
during seedling development, is reduced when in a stressed condition.

Soil Type

An acre foot of soil can contain about 1000 square miles of surface area
depending upon soil type. Fine textured soils or those having high clay
content have more surface area than course textured soils. Weed control
and crop injury occur at lower rates in sandy soils where less herbicide
is bound to the soil surface. Therefore, rate recommendations are often
based upon soil type. Organic matter also absorbs herbicides. Although
organic matter is characteristically low in desert soils, crop residues
can temporarily tie up Balan®, Kerb®, and Prefar® and reduce
both weed control and crop injury.

Herbicide Placement

Balan®, Kerb®, and Prefar® inhibit root and shoot growth
of germinating seeds. It is important that weed roots contact the herbicide
in the soil, and placement is critical. Concentrating these herbicides
in the area of the germinating lettuce seed will increase weed control
in that area but will also increase the potential for crop injury. Balan®
should be mechanically incorporated. Incorporation prior to listing beds
or deep incorporation will dilute the herbicide, increasing safety but
decreasing weed control. Shallow incorporation on shaped beds or incorporation
with irrigation water may result in unacceptable injury. Balan® is
strongly bound to the soil and will not move with irrigation water. Injury
will likely be temporary if roots are able to grow beneath the zone where
the herbicide is concentrated. Kerb® is normally applied after planting
and incorporated with sprinkler irrigation. When sprinklers are not used,
shallow preplant mechanical incorporation is recommended. Germinating
weed seedling roots must contact the herbicide and improved weed control
will be achieved if the herbicide is concentrated in the surface ½
inch of the soil. Unlike Balan® and Prefar®, Kerb® is not
strongly bound to soil and can leach below germinating weeds. Injury may
be prolonged if the crop roots are unable to grow beyond the herbicide.
Prefar® is normally applied preemergence after planting and incorporated
with sprinkler or furrow irrigation. It can also be applied by chemigation
through sprinkler systems. Best weed control is achieved when the herbicide
is incorporated in the top ½ inch of soil by sprinkler irrigation
or chemigation following planting. Potential injury is also increased
when this herbicide is concentrated around developing crop seedling roots.
Prefar® adheres to the soil and will not leach with irrigation, but
lateral movement has occasionally been observed across the bed. Injury
will likely be temporary if crop roots grow beneath the herbicide.

Movement in the Soil

The movement of herbicides in the soil is dependent upon absorption to
the soil, loss from photodecomposition, volatilization, and microbial
breakdown. It is not primarily dependent upon water solubility. Balan®
is volatile and subject to photodegradation and therefore should be incorporated
into the soil. Once incorporated, Balan® is strongly absorbed to the
soil particles. It will remain where it is incorporated throughout the
growing season. Prefar® is also strongly absorbed to the soil, if
left on the soil surface for several days will be slightly degraded by
sunlight. Prefar® has low volatility and like Balan®, will remain
where it is incorporated. Kerb® will normally not leach in the soil.
However, in the low desert, where it can be applied to dry soil and incorporated
with more than 6 inches of water, it can leach below germinating crop
and weed seeds. Kerb® is also somewhat volatile and can degrade by
photodecomposition if left on the soil surface for a prolonged time.

Injury Symptoms

Balan®, Kerb®, and Prefar® all kill weeds by inhibiting root
growth of developing seedlings. They all produce crop injury symptoms
that would be expected from plants that lack normal root growth, i.e.
stunting, water- and nutrient-stressed appearance. It can be difficult
to distinguish injury symptoms caused by these herbicides because of these
similarities and because they are sometimes used in combination. However,
there are differences among these herbicides that can produce distinctive
types of injury.

Balan®

Balan® is a dinitroaniline herbicide, one of the most important classes
of soil applied herbicides which includes Treflan®, Prowl®, Surflan®,
and others. These compounds stop the growth of roots and shoots of seedlings
by inhibiting cell division. Specifically, they interfere with the normal
movement of chromosomes during the mitotic sequence of cell division.
Balan® is taken up by the roots and shoots of developing seedlings
but is not translocated in the plant. The site of action is at or near
the site of absorption so the primary effect of Balan® is on the roots.
Roots that develop are "pruned", thick and short, giving them
a clubbed appearance (Figs. 1 & 2). Swelling of the hypocotyl or stem
below the cotyledons is common. Preemergence applications can result in
stem brittleness at the soil surface. The above ground portions of the
plant are stunted and often distorted (Fig 3). Balan® injury to lettuce
causes more distortion to the cotyledon and first true leaves than Prefar®
or Kerb®. Leaves are often thickened and distorted, resembling growth
regulator injury i.e. 2,4-D and dicamba (Fig. 4).

Figure 1

Figure 2

Figure 3

Figure 4

Kerb®

Kerb® is classified as a substituted amide, and like Balan®,
it inhibits root growth by interrupting the mitotic sequence of cell division.
However, Kerb® is absorbed through the shoots and roots and translocated
upward into the entire plant. Root growth is inhibited. Lateral roots
fail to develop but are not clubbed or pruned as they are with Balan®
(Fig. 5 & 6). The cotyledon and first true leaves are stunted and
thickened and sometimes, but not always, are chlorotic at the margins
(haloing) (Fig. 7). Lettuce is less likely to recover from severe Kerb®
injury than it is from Balan® or Prefar®. Seedlings will sometimes
wilt and die if injury is severe or adverse climatic conditions occur.

Figure 5

Figure 6

Figure 7

Prefar®

Prefar® is a substituted amide herbicide that inhibits cell division
and stops root growth. Only those roots that contact and absorb the herbicide
are effected and this herbicide does not move upward into the plant. Roots
that do not contact the herbicide grow normally and injured lettuce plants
are often able to recover if the tap root has the vigor to grow below
the treated soil. Injury symptoms sometimes appear irregularly along the
seed row if the herbicide is not uniformly distributed. Lateral roots
do not develop in the treated soil (Fig. 8). Clubbed or pruned roots are
not characteristic of injury from Prefar® (Fig. 9). Prefar® is
absorbed only by roots and stops cell division although the exact mode
of action is not well understood. Cotyledon and early true leaves are
stunted, sometimes thickened and malformed (Fig. 10).

Figure 8

Figure 9

Figure 10

Avoiding Injury

Injury from preemergence herbicides to lettuce can not always be avoided
and will occasionally occur due to factors that are not controllable.
There are steps that can be taken, however, that may minimize this occurrence.

Use lower rates on sandy or course textured soils.

Less herbicide is bound to the soil surface in course textured and low
organic matter soils. Crop roots will have greater exposure and be more
likely to be injured by herbicides in these soils. Recommended use rates
are lower in course textured soils.

Recalculate rates for band applications.

Recommended rates are given on a broadcast basis and it is necessary
to reduce them proportionately for band treatments. Failure to do so may
not be in compliance with the label and will increase the likelihood of
crop injury. The equation that should be used to determine the band application
rate is:

Avoid crop stress.

Stress created by uncontrollable environmental conditions can not be
avoided. Stress created by incorrect or suboptimal crop management is
avoidable and should be avoided when using preemergence herbicides in
lettuce. Anything that reduces crop vigor and prevents the roots from
growing through the treated zone, i.e. soil crusting, water stress, improper
fertility, or pest management, will increase the likelihood of herbicide
injury.

Herbicide placement can increase injury potential.

Concentrating the herbicide by tillage or water management, in the area
of the germinating crop seed, will provide the best weed control but will
also increase the possibility of crop injury. The grower should be prepared
to occasionally tolerate some crop injury when attempting to achieve maximum
weed control.

Test herbicides and leave untreated checks in your fields.

Testing these herbicides and combinations of these herbicides on a small
scale can determine injury severity. The only way to accurately assess
herbicide injury is to leave untreated checks. The time and effort used
to temporarily shut the sprayer off or to place a tarp on the ground is
often valuable. This should be done in 3 or 4 random locations across
the field and marked with stakes and flags.

Acknowledgement

We are grateful to Harry Agamalian, University of California (Ret.),
Dr. Peter Ellsworth, Dr. Jon Diehl, Kai Umeda, and Tim Knowles, The University
of Arizona for reviewing this bulletin.

Issued in furtherance of Cooperative Extension work, acts
of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture,
James A. Christenson, Director Cooperative Extension, College of Agriculture
and Life Sciences, The University of Arizona.

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